1. Field of the Invention
The present invention relates to a device that punches a sheet bundle obtained by bundling sheets sequentially delivered from an image forming device such as a copier or a printer and stacked and, more particularly, to a device capable of collectively punching a two-folded sheet bundle.
2. Description of the Related Art
There are widely known sheet processing device that align sheets delivered from an image forming device and staple the sheets or fold the sheets in a booklet form. Among these, some processing devices are configured to saddle-stitch the sheets using a staple or adhesive and then to fold them into a booklet form. In such devices, the sheets are subjected to punching one by one immediately after being discharged from the image forming device, followed by binding or center folding. However, in a case where the punching processing is performed before the binding or center folding, the punched positions may be displaced in a subsequent folded state.
To prevent the displacement in the punched position between the sheets, there is known a sheet processing device that performs binding or center folding first and then collectively performs the punching processing for the bound or folded sheets.
For example, Jpn. Pat. Appln. Laid-Open Publication No. 2011-84047 discloses a device that collectively punches punch holes in a saddle-stitched and folded sheet bundle.
In the above device, a saddle-stitching device, a three-side cutting device, and a punching device are connected to each other in this order in a sheet conveying direction. The punching device is configured to hold a sheet bundle by means of upper and lower block bodies and collectively punches punch holes in the sheet bundle using punching blades protruding from inside the upper and lower block bodies, respectively.
Further, Jpn. Pat. Appln. Laid-Open Publication No. 2013-99850 discloses a punching device that manually punches a center-folded sheet bundle, not automatically. This device uses a hollow cylindrical punching blade so as to punch a larger number of stacked sheets. Further, in this device, a waste discharging rod is fitted inside the hollow of the punching blade through a spring. The rod is configured to push punching waste that remains inside the hollow of the punching blade after the punching processing outside the punching blade. That is, the waste discharging rod is pressed against the stacked sheets while the punching blade punches the stacked sheets, whereby the punching waste is pushed outside the punching blade.
The device that performs the collective punching processing for the sheet bundle, especially, a folded sheet bundle or device that pushes punching waste in the punching blade has the following problems.
First, the device disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2011-84047 is configured to perform the collective punching processing for the folded sheet bundle. Thus, the punched positions of the sheets are not displaced to thereby provide a better finish than in a case where the conventional method in which the punching processing is performed one by one for the sheets delivered from an image forming device. However, larger force needs to be applied to the punching blade in order to collectively punch the sheet bundle in which a plurality of sheets are stacked. For example, a force of about 50 kg is required to punch a 20-page sheet bundle obtained by folding 10 sheets in the center, and the larger the number of sheets constituting the sheet bundle, the larger this force should be. This involves increase in size and stiffness of the device and increase in drive force.
To cope with this problem, as disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2013-99850, the punching blade is formed to have a hollow cylindrical structure. This can reduce resistive force of the sheets against the punching blade, whereby power for punching the sheets can be saved. In addition, the punching waste remaining in the hollow portion of the punching blade can be discharged by means of the discharging rod.
However, the discharge rod disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2013-99850 is fitted to the punching blade through a spring and operates integrally with the punching blade. Therefore, even though the punching blade is formed into the hollow shape to reduce penetration resistance against the sheets, the punching blade presses the sheets together with the discharging rod in the middle of the punching operation, the resistive force of the sheets is not reduced sufficiently. Further, when the sheets to be punched are thin papers or when the number of sheets to be punched is small, the discharging rod pushed by the spring pushes the sheets in the middle of the punching operation, so that a residue of punching waste is caused or a punching shape does not become constant. When force of the spring interposed between the punching blade and discharging rod is reduced so as to prevent the above situation, the punching waste remaining in the hollow portion of the punching blade cannot be discharged sufficiently.